Cheaper Plastic Solar Cells in the Works

South Dakota State University’s Diane Hinkens describes her work in an interdisciplinary research collaboration that is trying to design, synthesize and eventually fabricate a more efficient and less costly solar cell

Sunlight, or solar radiation, is a remarkable phenomenon. It is the energy source in photosynthesis, it makes us warm on summer days, and, if future solar cells can be made more efficient and less costly, it may be our best source for reliable, clean and renewable energy.

As a postdoctoral researcher in the South Dakota State University (SDSU) department of electrical engineering, I am working with my advisor, Qiquan Qiao, an assistant professor in the department's Center for Advanced Photovoltaics, and Seth Darling, an assistant scientist at the Department of Energy's Argonne National Laboratory Center for Nanoscale Materials, to design, synthesize and eventually fabricate a more efficient and less costly solar cell.

We are trying to develop a cell that addresses the main challenge facing solar energy devices: how to absorb more of the sun's energy for electricity production.

Plastic Cells

Unlike the vast majority of today's solar cells, which are expensive because they are made from silicon-based, or inorganic, semiconductors, the solar cell we are creating will be less costly as it will be made from organic, or carbon-based, semiconductors made from polymers. We will use two different types of polymers: one that is electron-deficient, an organoborane polymer, and one that is electron-rich, a thiophene polymer.

Since my skills are focused on creating organoborane molecules--molecules made from carbon, boron and hydrogen--I rely heavily on Qiao, who is an expert in the physics of solar cells, and Darling, who is an expert in self-assembly and chemical computation. As we each have expertise in specialized areas of science, putting our ideas together to make this project work is really a great interdisciplinary research collaboration!

The organoborane and thiophene polymers are chain-like molecules made from carbon, boron, sulfur and hydrogen. These polymers have alternating double bonds and are flat--characteristics necessary for electrons to travel through the backbones of the polymers and produce electricity.

When we connect the organoborane and thiophene polymers, each with opposing electronic properties, we will create a "molecular p-n junction," key to collecting and using electrons for electricity.

By synthesizing the p-n junction within the polymer molecules, we hope to overcome many of the limitations of current organic solar cells.

Additionally, we will incorporate molecules known to absorb different wavelengths of light into the polymer chains. By doing this, we hope that these polymers will be able to absorb nearly the entire spectrum of visible light, which has wavelengths ranging from approximately 400 nanometers (violet light) to 750 nanometers (red light), thereby harnessing much more of our sun's energy.

Self-assembling Arrays

One reason my collaborators and I chose to research these polymers is because they may be capable of self-assembling in just a few seconds to form very tiny, ordered arrays of materials. The arrays will allow the electrons to more easily find their path out of the ordered blocks to produce electricity.

The proposed solar cell would consist of approximately 1015, or one quadrillion, polymer molecules enclosed in an area of just one square centimeter.

The immediate goal for our research project is to develop a greater understanding of this class of materials, knowledge that will be used down the line to develop and fabricate our proposed solar cell.

Printing Cells

To identify which polymer structures best fit our requirements, I have been using a commercial computational chemistry software program that will give me direction for the biggest challenge of this project, which will be making and characterizing these polymers.

These solar cells may be inexpensive to produce because the organic polymers can be created using low-cost techniques like reel-to-reel processing, similar to the method behind newspaper printing, which results in a material that is lightweight and mechanically flexible.

In September 2008, I was awarded an inaugural National Science Foundation American Competitiveness in Chemistry Fellowship, a two-year grant that is giving me an opportunity to both contribute to U.S. competitiveness and to involve students from traditionally underrepresented groups in this important area of science. I plan to develop a solar cell laboratory for the Chicago Science Alliance, which supports science teachers in the Chicago Public Schools, and to work with my advisor to develop hands-on activities describing solar cell materials for inclusion in a mobile science laboratory. The lab would travel across South Dakota to reach small rural schools as well as schools on Native American reservations.